The present invention generally relates to a four-wheel locking system for a motor vehicle and more particularly to an arrangement to preventing locking of the system when the vehicle must start under a heavy load.
A four-wheel locking system is disclosed by German patent specification (DE-PS) 35 07 490 wherein a planetary transmission includes planet wheels disposed on a planet carrier mate with an input sun gear and an output sun gear which is coaxially arranged next to it and has a slightly lower number of teeth. This arrangement is possible by means of corresponding profile corrections of the gear wheels. The ratio for the direct drive-through of the input sun gear to the output sun gear deviates only slightly from 1:1; however, the gear wheel ratio of the input sun gear to the planet carrier is much higher, specifically 18:1. If a rotational speed difference occurs at the locking system, that is, between the input shaft and the output shaft, the planet carrier, corresponding to the high ratio, will rotate at a high rotational speed.
A braking device applied to the planet carrier of this arrangement therefore must generate only a slight braking torque in order to lock the planetary drive and thus transmit driving torque from one vehicle axle to the other vehicle axle. Occasionally, difficulties occurred when a motor vehicle having such a locking system must, for example, start on a hill under a high load. In the planetary transmission, this may result in a self-locking as a result of excessive friction losses, mainly in cases where normal cut gear wheels are used which have the conventional coefficients of roughness of approximately 1 .mu.m.
It is an object of the present invention to provide gearings of a planetary gear system such that self-locking is avoided, without the requirement of costly manufacturing processes, such as the burnishing or lapping of the gear wheels.
In order to achieve these and other objects, the real pitch module (m) of all gear wheels is proportional to the driving torque to be transmitted and is dimensioned to be small and the operational pressure angle .alpha. is as large as possible. Thus, from the large number of parameters which influence the tooth friction losses, the real pitch module m is selected for an optimization. Corresponding to the driving torque to be transmitted, it is dimensioned as small as still acceptable with respect to strength. As a supplementary measure, in order to ensure sufficient root strength of the teeth and load capacity of the flanks despite the small module, the operational pressure angle .alpha. is selected to be as large as possible with respect to overriding constructive requirements.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.